Power control for a low power display

ABSTRACT

A low power display device including a power control circuit for controlling power from an environmental energy source to a power storage device that is charged by the environmental energy source, is described.

BACKGROUND

This specification relates to information presentation.

In office buildings of some agencies and corporations, conference roomschedules are printed on paper and posted daily. These paper schedules,while accurate at the time of posting, are sometimes inaccurate within afew hours. Furthermore, the amount of paper used and the human effortrequired to manually post daily paper schedules can be significant foran organization with a large number of conference rooms. Another exampleof potentially wasteful paper usage includes regular printing of a menuthat changes daily. Generally, the periodic posting of informationsubject to change using paper can be quite inefficient.

Electronic display devices exist as alternatives to daily paper postingof conference room schedules and periodic posting of other information.However, these conventional devices can be expensive to purchase andinstall. For example, some conventional devices require investment inproprietary operating systems and applications, which may not becompatible with an organization's existing scheduling application.Additionally, these conventional devices typically require a wirednetwork connection (e.g., an Ethernet connection) and a wired powerconnection (with or without a battery backup) that continuously suppliespower to the devices. These requirements of conventional electronicdisplay devices can render the devices impractical and cost-prohibitivefor certain applications, such as those described above.

SUMMARY

This specification describes technologies relating to low power displaydevices and systems.

In general, one aspect of the subject matter described in thisspecification can be embodied in methods including periodically samplinga voltage of a power storage device, determining if the voltage isgreater than a turn off voltage, and if the voltage is less than a turnon voltage, causing a switch to set the power storage device to acharging state, the power storage device being charged by anenvironmental energy source while in the charging state, wherein asampling period for periodically sampling the voltage of the powerstorage device while the power supply device is in a discharging stateis less than an amount of time required for the voltage of the powerstorage device to decrease from the turn on voltage to a minimum inputvoltage of a power supply powered by the power storage device.

In general, another aspect of the subject matter described in thisspecification can be embodied in a power control circuit that includes acapacitor; a switch circuit having two positions configured toselectively connect a photovoltaic cell to the capacitor and to outputposition information indicating a current position of the switch; and aprocessing circuit configured to periodically read a voltage across thecapacitor and to read the position information from the switch circuitand to set a position of the switch circuit based on the voltage acrossthe capacitor and the position information.

The power control circuit can be configured to have a sampling periodfor reading the voltage across the capacitor having a duration of lessthan an amount of time required for the capacitor to discharge from aturn on voltage to a minimum input voltage, the minimum input voltagebeing a voltage required for proper operation of a power supply (e.g., acharge pump) drawing power from the capacitor.

The power control circuit can be configured to have a sampling periodfor reading the voltage across the capacitor having a duration of lessthan an amount of time required for current from the photovoltaic cellto charge the capacitor from a turn off voltage to a voltage capable ofdamaging the capacitor.

The processing circuit can be configured to maintain a low power statewhen not reading the voltage across the capacitor or when not performingoperations to set a position of the switch circuit based on the voltageacross the capacitor. The processing circuit can be configured to setthe switch circuit to an on position if the voltage across the capacitoris less than the turn on voltage. The processing circuit can beconfigured to set the switch to an off position if the voltage acrossthe capacitor is more than a turn off voltage.

Particular embodiments of the subject matter described in thisspecification can be implemented to realize one or more of the followingadvantages. Paper conference room displays or other general displaysbased on printed paper, grid powered display devices, and batterypowered display devices can be replaced with a wireless, low powerdisplay device that is automatically updated as needed to display up todate information (e.g., throughout the day). The display device ispowered by indoor solar energy stored by a capacitor, which eliminatesthe need for a wired power connection (with or without a batterybackup). The low power display device saves paper and in the case ofconference room schedules replaces the manual process of physicallydelivering paper schedules to each conference room. Additionally, thelow power display device eliminates the need for batteries which due totheir chemical properties can pose environmental risks, and the need toperiodically change batteries, as required in battery powered displaydevices, and also does not require a connection to a power grid. Use ofthe low power display can avoid costly infrastructure updates, theinstallation of additional power outlets, for example.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example low power display system.

FIG. 2A is an illustration of an example low power display device of thelow power display system of FIG. 1.

FIG. 2B is a block diagram of an example low power display device of thelow power display system of FIG. 1.

FIG. 2C is an illustration of example alternative energy sources of alow power display device.

FIG. 3A is a block diagram of an example power control circuit of thelow power display device of FIG. 2B.

FIG. 3B is a schematic of an example charge control circuit of the lowpower display device of FIG. 2B.

FIG. 3C shows an example graph of a decreasing voltage across a powerstorage device.

FIG. 3D shows an example graph 482 of an increasing voltage across apower storage device.

FIG. 4 is a flow diagram of an example process for operating the chargecontrol circuit of FIG. 3B.

FIG. 5 is a flow diagram of an example process for updating a schedulefor a conference room.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION Example Low Power Display System

FIG. 1 is a block diagram of an example low power display system 100.The example low power display system 100 includes a server 110, basestations 120, and low power display devices 130. Although the examplelow power display system 100 is illustrated with one server 110, threebase stations 120, and nine low power display devices 130, the low powerdisplay system 100 can include multiple servers 110, where each server110 is connected to one or more base stations 120, and each base station120 is connected to one or more low power display devices 130.

The server 110 serves as the central control for the low power displaysystem 100. The server 110 can be implemented entirely in software,e.g., a server application. The low power display system 100 can bedesigned to utilize the server 110 for the majority of the system'sprocessing needs. This, in turn, allows the base stations 120 and thelow power display devices 130 to be less complex and implemented withless expensive components than if the system's processing wasdistributed more evenly across all the devices.

The server 110 coordinates the mapping of the low power display devices130 to the base stations 120. In some implementations, the mapping of alow power display device 130 to a particular base station 120 is basedon the radio signal strength of the low power display device 130. Suchradio signal strength mapping techniques can be described in Part 15.4:Wireless Medium Access Control (MAC) and Physical Layer (PHY)Specifications for Low-Rate Wireless Personal Area Networks (WPANs),Institute of Electrical and Electronics Engineers (IEEE) Std802.15.4-2006. Other mapping techniques can also be used. As part ofmanaging the distribution of low power display devices 130 among thebase stations 120, the server 110 can, in some implementations, alsohandle failover. That is, when a base station 120 fails, the server canre-allocate the mapping of the low power display devices 130 connectedto the failed base station 120 to a functional base station 120.

In some implementations, the server 110 collects, aggregates, andexports status information and logs for the base stations 120 and thelow power display devices 130. The status information collected caninclude, for example, the remaining power of a low power display device130, the radio signal strength of a low power display device 130, thestatus messages passed from a low power display device 130 to a basestation 120, and the frequency that a low power display device 130communicates with a base station 120 or a base station 120 communicateswith the server 110.

In some implementations, the server 110 is in communication with thebase stations 120 over a network. In some implementations, the networkis a local area network (LAN). For example, the base stations 120 canhave a wired connection to the server 110 by an Ethernet connection,i.e., the IEEE 802.3 protocol. Alternatively, the base stations 120 canbe wirelessly connected to the server 110 on a wireless LAN network,e.g., a Wi-Fi network standardized by IEEE 802.11.

In some implementations, the low power display system 100 can providereal-time, updated conference room scheduling information throughout theday. The updated conference room scheduling information can bedistributed to the low power display devices 130 for display as areplacement for posted paper schedules. Other types of text or imageinformation can alternatively be displayed using the low power displaysystem 100. For example, the low power display devices 130 can displaymenus, rosters, advertisements, or any other information that can beaccommodated by the display of the low power display devices 130. Thelow power display devices 130 can also display multiple types ofinformation simultaneously or throughout the day. Further example usesof the low power display system 100 include the posting of generalreservation information for resources, e.g., gym equipment andfacilities such as weight machines, exercise machines, racquetballcourts, and basketball courts.

In some implementations, in addition to displaying updated schedulinginformation for a particular conference room, a low power display device130 can also display information about one or more nearby conferencerooms. For example, the low power display device 130 can display a mapof the floor indicating the particular conference room and the closestconference room that is currently available.

In some implementations, the low power display system 100 can distributea particular type of data to a particular low power display device 130depending on the location of the particular low power display device130. For example, a first low power display device 130 located outside acafeteria can receive menu information for display, while a second lowpower display device 130 located outside a conference room can receivethe conference room's schedule for display.

In some implementations, a calendar application is integrated with ane-mail application, and the low power display device 130 associated withthe office or cubicle of a particular user of the e-mail application canbe posted outside the user's office or cubicle to display the user'scalendar schedule.

Some organizations manage conference room schedules using a calendarapplication, where each conference room is a resource in the calendarapplication. Typically, individuals of the organization can use aninterface of the calendar application to reserve a conference room for aparticular meeting or event. Calendar data for the calendar applicationcan be maintained on one or more servers remote to the low power displaysystem 100.

If the low power display system 100 is used to provide conference roomscheduling information, each low power display device 130 can beassociated with a particular conference room that is a resource in thecalendar application. The low power display device 130 can be locatedoutside the associated conference room (e.g., attached by Velcro to awall or door of the conference room). Typically, several base stations120 can be placed on each floor of a building to provide coverage forall the conference rooms on the floor.

Each base station 120 is associated with one or more low power displaydevices 130 as determined by the mapping by the server 110. A basestation 120 can retrieve display information for the low power displaydevices 130 to which the base station 120 is mapped. For example, thebase station 120 can retrieve calendar data for the conference roomassociated with a particular low power display device 130. In someimplementations, the calendar data is retrieved from the remote serversusing an application programming interface (API) of the calendarapplication. The base station 120 can process the retrieved calendardata to generate instructions for rendering the calendar data as animage sized for the display of the low power display device 130. Theseinstructions are then transmitted to the particular low power displaydevice 130. In some implementations, data for one or more low powerdisplay devices 130 is cached at base stations 120 mapped to those lowpower display devices 120.

The base station 120 is in communication with the associated low powerdisplay devices 130 over a wireless network. In some implementations,the wireless network is a wireless PAN. For example, the base station120 can be wirelessly connected to the associated low power displaydevices 130 on a wireless PAN based on the IEEE 802.15.4 protocol in theunlicensed 2.4 GHz spectrum. The IEEE 802.15.4 protocol works well forubiquitous communication between devices within a wireless PAN, becausethe protocol delivers low-cost and low-speed communication. In addition,the protocol can be implemented in transceivers capable of communicationat low power levels.

In some implementations, the base station 120 receives requests from oneor more associated low power display devices 130 for displayinformation. Exponential backoff can be used when the base station 120is unavailable. For example, if a low power display device 130 transmitsa request for display information to a base station 120 and the basestation 120 is not responsive, the low power display device 130 canresend another request after a predetermined period of time has past.The periods of time between repeated requests can increaseexponentially.

The base station 120 can be implemented as part software and parthardware. In some implementations, the base station 120 is a universalserial bus (USB) peripheral device coupled to a computer. In someimplementations, the operating system of the computer runs a daemon thatlistens to the associated low power display devices 130 for requests toupdate display information (e.g., calendar data). The base station 120includes a transceiver (e.g., a transceiver configured to support IEEE802.15.4 protocol connectivity) to communicate with the associated lowpower display devices 130 on the wireless network.

In some implementations, the base station 120 is a smallmicrocontroller-based system that does not need to be coupled to acomputer for processing. In these implementations, some of theprocessing of the base station 120 can be shifted to the server 110, andthe cost of the base station 120 can be significantly lower than thecost of the base station 120 in the USB peripheral deviceimplementation. The microcontroller-based system can include, forexample, a microcontroller, a small amount of storage, a LAN networkinterface, and a wireless PAN transceiver. The storage can be used forcaching display data and log records.

Each low power display device 130 is associated with a particular basestation 120 as determined by the mapping by the server 110. In order tocommunicate with the associated base station 120, the low power displaydevice 130 includes a transceiver for the wireless network (e.g., atransceiver configured to support IEEE 802.15.4 protocol connectivity).The low power display device 130 uses the transceiver to request andreceive display information from the associated base station 120. Thelow power display device 130 then renders the display information inaccordance with instructions received by the associated base station120. If the display information is calendar data for an associatedconference room, an identifier (e.g., a MAC address) for the low powerdisplay device 130 can be used as a unique identifier for the associatedconference room. The low power display device 130 is further describedbelow in reference to FIGS. 2A-4.

Example Low Power Display Device

FIG. 2A is an illustration of an example low power display device 200 ofthe low power display system 100 of FIG. 1, and FIG. 2B is a blockdiagram of an example low power display device of the low power displaysystem 100 of FIG. 1. Although the example low power display device 200is described in the context of a calendar data display system, othertypes of image or text data can alternatively be displayed. In someimplementations, audio data can be provided in addition to or in lieu ofdisplay data. In some implementations, the low power display device 200can be assembled by using off-the-shelf hardware components.

The low power display device 200 is an indoor solar powered, low powerconsumption display unit with wireless radio network connectivity. Thelow power display device 200 is designed to use small amounts of power,allowing it to be powered solely by solar energy collected by a solarpanel 210 of the low power display device 200. The solar panel 210 ofthe low power display device 200 is capable of collecting any source oflight energy, including indoor lighting. Although this indoor solarenergy or ambient energy typically provides much less energy than normaloutdoor solar energy, the solar energy collected from normal indoorlighting is sufficient to power the low power display device 130 withoutthe need for a battery or a wired power connection.

FIG. 2C depicts example alternative energy sources of a low powerdisplay device 130. In some implementations, the low power displaydevice 200 is powered by other passive power supplies or environmentalpower sources (e.g., a power source that takes, creates, or derivesenergy from its environment), which do not draw power from a connectionto a power grid. For example, the low power display device 200 canalternatively be powered by parasitic energy, energy generated byopening and closing a door, energy collected from ambient radiofrequency noise, or other energy sources. Energy can be obtained from amoving door, for example, through a DC motor turned through gearing bythe motion of the door. An example for obtaining energy from a movingdoor is shown in FIG. 2C. Motor and gearbox 314 are turned by motion ofthe conference room door with respect to the doorframe. Alternatively,the gearbox 314 can be mounted to the doorframe instead of the dooritself.

In some implementations, a hand crank such as the hand crank 318 shownin FIG. 2C or other user actuated device can be provided with the lowpower display device 200 so that a user can add power to the device asneeded to view up to date information.

In some implementations, power can be supplied by a piezoelectric shock.For example, a piezoelectric mat 316 can be placed in front of or near alow power display device 200 so that a user stepping on the mat produceselectricity due to a piezoelectric effect.

The low power display device 200 includes a power control circuit (suchas the power control circuit 304 of FIG. 2B), which selectively connectsand disconnects a power storage device 305 (e.g., a capacitor) from thesolar panel 210, and a processing device (such as the processing device308 of FIG. 2B), which manages power consumption in the low powerdisplay device 200 and communicates with the associated base stationthrough the transceiver. Both the power control circuit 304 and theprocessing device 308 (e.g., a microcontroller) are powered by the powerstorage device 305 and are described in more detail below.

The power storage device 305 stores energy from the solar panel 210. Insome implementations, the power storage device 305 (e.g., anultracapacitor) is selected to provide sufficient charge storagecapacity to eliminate the need for including a battery in the low powerdisplay device 200. The low power display device 200 also includes atransceiver (such as the transceiver 312 of FIG. 2B), which is poweredby the power storage device that is local to the low power displaydevice (e.g., an on board capacitor) and is used for communicating withthe associated base station, as described above. The antenna 220 for thetransceiver is illustrated in FIG. 2A.

A display device 230 of the low power display device 200 is connected tothe processing device 308 and is powered by the power storage device(e.g., a capacitor). In the example of FIG. 2A, the display device 230displays a rendered image of calendar data for the associated ConferenceRoom A. The displayed calendar data includes the date, time, andorganizer of events scheduled for Conference Room A, although otherinformation can be additionally or alternatively displayed.

In some implementations, the display device 230 maintains a persistentrendered image of the display information without requiring additionalpower. For example, the display device 230 can be a cholesteric liquidcrystal display (LCD). Traditional LCDs require power at all times tomaintain an image. This amount of required power is generally more powerthan can be stored in the power storage device 305 of the low powerdisplay device 200. A cholesteric LCD allows the displayed image to stayfixed on the display indefinitely regardless of whether or not power issupplied to the low power display device 200. This type of persistentdisplay is referred to as a bistable display. In some implementations,the display device 230 consumes about a half a joule of power whenredrawing the full screen.

In some implementations, an additional solar panel can be hidden behindthe display device 230. For example, some cholesteric LCDs allowapproximately 50% reflectivity, which means about 50% of energy istransmitted through the cholesteric LCD to the background. If anadditional solar panel is placed behind the display device 230,additional solar energy can be collected for storage locally in thepower storage device (e.g., in a capacitor).

In some implementations, the solar panel 210 of the low power displaydevice 200 is located behind the display device 230. In theseimplementations, the display device 230 occupies substantially theentire front surface of the low power display device 200. If the lowpower display device 200 includes a manual actuator 240 (as describedbelow), the manual actuator 240 can be located on one side of the lowpower display device 200. Locating the solar panel 210 completely behindthe display device 230 minimizes the footprint of the low power displaydevice 200 while maximizing the space available for collecting power.

In some implementations, the low power display device 200 operates in anultra-low power sleep mode. A cholesteric LCD can maintain the finalimage state even in sleep mode. In some implementations,microcontrollers of the low power display device 200 perform noprocessing while in sleep mode other than a hardware counter that wakesthe low power display device once a sleep period has expired. Forexample, prior to entering the sleep mode, software of themicrocontroller can indicate a duration of a sleep period that is usedby the hardware counter to determine when to wake the device. In someimplementations, a base station 120 can indicate the duration of a sleepperiod to a low power display device mapped to that base station 120.

The processing device can cause the low power display device 200 toenter the sleep mode. In some implementations, the low power displaydevice 200 periodically wakes up to transmit a request to the associatedbase station for an update (e.g., for updated display information). Insome implementations, the duration between periodic wake modes can bedetermined algorithmically. For example, the processing device cancondition transmission of a request for display information based on thelevel of a charge in the power storage device 305. In someimplementations, the duration between periodic wake modes is determinedby a predictive algorithm.

In the example of FIG. 1, the low power display device 200 includes amanual actuator 240 (e.g., a hardware button). This manual actuator 240can generate an actuation signal when actuated (e.g., depressed) by auser. The actuation signal can cause the processing device to awake thelow power display device 200 from the sleep mode and cause thetransceiver to wake up. In some implementations, wake up can include thedisplay of information and/or the transmission of a request for displayinformation. In some implementations, the charge stored in the capacitoris used approximately 2-3 seconds every hour to update the image of thedisplay device 230 after updated display information is received.

In some implementations, the display device 230 can include an interfacethat allows a user to select one or more options by, for example,actuating the manual actuator 240. For example, if the display device230 is used to display calendar data, an interface can allow a user toconfirm attendance at an event, cancel an event, or extend the durationof an event. In some implementations, actuations of the actuator 240 arereported to the base station 120 to which the display device 230 ismapped. Processing performed in response to the actuation can beperformed by the base station 120 and/or the server 110.

FIG. 2B shows a block diagram of an example low power display device300. An output from the environmental energy source 302, a photovoltaiccell, for example, is connected to a power control circuit 304. Thepower storage device 305, (e.g., capacitor) of the power control circuit304 collects energy from the environmental energy source 302 forpowering the low power display device 200. Output from the power controlcircuit 304 is provided to the power supply circuit 306. Using powerfrom the power control circuit 304, the power supply circuit 306 (acharge pump, for example) can provide one or more voltage outputs foruse in the low power display device 200. The power supply circuit 306can include, for example, one or more charge pump power supplies.

The power supply circuit 306 supplies power to a processing device 308,a display device 230, a transceiver 312, and in some implementations canprovide power back to the power control circuit 304 for controlfunctions of that circuit as described below. Power provided to thepower control circuit 304 can be used, for example, by control logicand/or one or more processing devices of the power control circuit 304.The control logic and/or one or more processing devices of the powercontrol circuit can, for example, control the flow of energy from theenvironmental energy source 302 to protect the low power display device200. More specifically, the control logic and/or one or more processingdevices of the power control circuit 304 can regulate the amount ofcharge collected in the power storage device 305 to protect the powerstorage device 305 from damage due to the accumulation of too high acharge (across a capacitor, for example).

In some implementations, the power control circuit 304 is configured ina default mode to allow current to flow from the environmental energysource 302 to the power supply circuit 306 when there is no power beingsupplied to the power control circuit 304 from the power supply circuit306. Such a condition would be the case, for example, at an initialpower up of the low power display device 200. After a transient periodfollowing power up of the lower power display device 200, the powersupply circuit 306 will begin providing power to the power controlcircuit 304 which can then begin to monitor and control energycollection from the environmental energy source 302.

Power from the power supply circuit 306 is supplied to the processingdevice 308. The processing device 308 can interface with and receivedata from the transceiver 312 for communication with one or more basestations 120 and can process the data for the display device 230 asdescribed above.

Example Power Control Circuit

FIG. 3A includes a block diagram 400 of an example power control circuit304. A voltage output of a photovoltaic cell 401 is connected to aswitch circuit 404. In a closed position, the switch circuit 404connects the voltage output from the photovoltaic cell 401 to anungrounded lead of a capacitor 406 and to an input of the power supplycircuit 306. While the switch circuit is in a closed position thecapacitor 406 accumulates an electrical charge. While the switch circuit404 is in an open position, the photovoltaic cell 401 is disconnectedfrom the capacitor 406 and the charge on the capacitor 406 is drained bythe power supply circuit 306. Operation of the switch circuit iscontrolled by the processing circuit 402. The processing circuit 402 canreceive power, a regulated voltage source, for example, from the powersupply circuit 306.

The processing circuit 402 can monitor the voltage across the capacitor406 at an input of the processing circuit 402 through, for example, aresistor 408. The processing circuit 304 can control the operation ofthe switch circuit 404 through the switch control line 410. A presentstate of the switch (i.e., whether the switch circuit is in an open orclosed position) is supplied to the processing circuit 402 through theswitch feedback line 412. The processing circuit 402 controls operationof the switch circuit 404 to maintain an input voltage to the powersupply circuit 306 that is sufficient for power functioning of the powersupply circuit 306 without damaging the capacitor 406. If the voltageacross the capacitor 406 becomes too high (e.g., reaches a turn offvoltage), the processing circuit can direct the switch circuit 404 toopen in order to protect the capacitor 406 from damage. If the voltageacross the capacitor becomes too low (e.g., reaches a turn on voltage),the processing circuit can direct the switch circuit 404 to close tocharge the capacitor 406 using power from the photovoltaic cell 401.

FIG. 3B is a schematic of an example charge control circuit 304 of thelow power display device 200. The power control circuit includes anon/off switch 452 that can be, for example, manually actuated by a userto switch the lower power display device on or off. In this example, theswitch 452 is a double pole, double throw switch and is shown in an off(open) position. When the low power display device is switched on,electrical current flows from the photovoltaic cell 401 through a diode450 (e.g., a schottky diode) to a (e.g., 10 farad) ultracapacitor 453and to the power supply circuit 306. The diode 450 prevents current flowfrom the ultracapacitor 453 to the photovoltaic cell 401, in, forexample, low-light conditions.

The switch circuit 404 of FIG. 3A is implemented in the schematic ofFIG. 3B using a double pole double throw relay including coil element454, pole 456 and pole 458. In some implementations, the relay is abistable relay that maintains the last position to which it has beendirected even after power has been removed from the coil. The use of abistable relay reduces the power required to operate the voltage controlcircuit and thus reduces the overall power consumption of the low powerdisplay device 200. The relay is shown in a closed position that allowscurrent from the photovoltaic cell 401 to power the power supply circuit306 and to charge the capacitor 453. In the closed position, the pole456 connects diode 450 through the power switch 452 to the node labeledVCAP 460 (which is shown connected to the input of the power supplycircuit 306, to the capacitor 453, and to the resistor 464. When therelay is in an open position, the pole 456 disconnects the photovoltaiccell 401 from the VCAP node 460.

The pole 458 provides relay position feedback to the microcontroller 462which implements the processing circuit 402 of FIG. 3A. When the relayis in a closed position, the pole 458 connects a pin of themicrocontroller 462 to a node VCC 464 which can, for example, beconnected to a regulated voltage output of the power supply circuit 306.When the relay is in an open position, the pole 458 connects this pin ofthe microcontroller 462 to ground. The microcontroller 462 (a PIC16F506,for example) can read a voltage at the pin connected to the pole 458 todetermine a position of the relay.

The relay is actuated by the microcontroller 462. In the example shown,one side of the coil 454 is connected to three pins of themicrocontroller 462 and the other side of the coil 454 is connected tothree other pins of the microcontroller. Three pins are connected toeither side of the coil to add the respective amount of current that themicrocontroller can flow through each pin to reach the amount of currentneeded to flow through the coil 454 to actuate the relay. To actuate therelay, a logic 0 can be set at the three pins connected to one side ofthe coil 454 while a logic 1 is set on the three pins on the other sideof the coil 454. Current flows from the pins set to logic 1 to those setto logic 0 through the coil 454 to set a state of the relay (closed forexample). Flipping the logic state of the pins connected to the coil 454will reverse the current flow to set the other state of the relay (openfor example).

The microcontroller 462 monitors the voltage across the capacitor 453through the resistor 464 and actuates the relay in order to protect thecapacitor 453 from damage due to too high of a potential across itsterminals. To conserve power the microcontroller 462 can be configuredto maintain a state of low power consumption and leave that stateperiodically to sample the voltage across the capacitor 453, causingcurrent to flow through the coil 454 only when needed to change theposition of the relay from its present state as indicated by thefeedback provided from pole 458. For example, the microcontroller cansample the voltage across the capacitor 453 four times per second, andonly acting on that information (i.e., actuating the relay) if the relayneeds to be switched from its current position.

In some implementations, a dual coil bistable relay can be used in placeof the single coil relay described above.

In some implementations, the time interval between voltage samples (andany resulting changes made to the state of a switching circuit) is set,and a turn off voltage is selected so that a power storage device (e.g.,a capacitor) can not be charged (by the environmental energy source(e.g., solar panel or photovoltaic cell) from the turn off voltage to avoltage that will damage it during the time between samples.

In some implementations, the time interval between voltage samples (andany resulting changes made to the state of a switching circuit) is set,and the turn on voltage is selected so that the power storage device cannot be discharged from the turn on voltage to a minimum input voltage ofthe power supply circuit 306 during the time between samples. Theminimum input voltage of the power supply circuit can be, for example, alower limit below which the power supply circuit 306 will not functioncorrectly.

In some implementations, the power control circuit 304 is designed toprovide a window of hysteresis within which the switch circuit canremain in a given state for a period of time. Leaving the switch circuitin a given state (as opposed to rapidly transitioning the switch circuitbetween states to maintain the voltage across the capacitor) conservespower by allowing the processing circuit 402 to remain in a low powerstate and by avoiding the use of any power required to actuate theswitch circuit.

For purposes of example, a capacitor used as a power storage device canhave a rated voltage of 2.7 volts. In this example, the capacitor can bepotentially damaged by voltages at some level higher than 2.7 volts(e.g., a maximum rated voltage limit of 3 volts). 2.7 volts can be usedas the turn off voltage, and 2.5 volts can be used as the turn onvoltage. For low power display devices using a photovoltaic cell,charging times from 2.5 volts to 2.7 volts depend on an amount ofambient light. In full sunlight conditions charging from 2.5 to 2.7volts can take less than one second. In dim light the charging time canbe several minutes. Draining the capacitor from 2.7 volts to 2.5 voltsgenerally takes more time, on the order of hours if the low powerdisplay device is in a sleep mode and no updates are occurring. Drainingthe capacitor from 2.7 volts to 2.5 volts can occur on the order ofminutes if the display is being updated frequently. The voltage of thecapacitor can be sampled, for example, every 250 milliseconds (4 timesper second).

In some implementations one or more solid state devices, such as metaloxide semiconductor field effect transistors (MOSFETs) are used toswitch the charging of the capacitor on and off and to provide currentstate feedback to a processing circuit.

FIG. 3C shows an example graph 470 of a decreasing voltage across apower storage device. Line 472 represents a voltage across a powerstorage device (e.g., a capacitor) as the voltage is drained over time.Line 474 represents a turn on voltage of a power control circuit, andline 476 represents a minimum input voltage of a power supply drawingpower from the power storage device. A processing circuit of the powercontrol circuit can monitor the voltage across the power storage deviceand cause charging of the power storage device to begin if the monitoredvoltage is less than the turn on voltage. The dashed line 478 representsa voltage across the power storage device after the processing circuithas initiated charging of the power storage device.

If the voltage across the power storage device is allowed to decreasebelow the rated minimum input voltage of the power supply (e.g., acharge pump), the output of the power supply can drop as shown by line480, for example. The sampling period for sampling the voltage acrossthe power storage device and the turn on voltage can be selected so asto avoid a drop in the power supply output from occurring. For a givenminimum rated voltage input to a power supply and turn on voltage, themaximum time between samples (shown as Tmax1 in FIG. 3C) is the timerequired for the voltage across the power storage device to be drainedfrom the turn on voltage to the rated minimum input voltage of the powersupply.

FIG. 3D shows an example graph 482 of an increasing voltage across apower storage device. The line 484 represents the voltage across a powerstorage device. The voltage is shown as increasing while the powerstorage device is being charged. Line 486 represents a turn off voltageof a power control circuit, and line 488 represents a maximum ratedvoltage of the power storage device (beyond which damage to the devicecould occur, for example). A processing circuit of the power controlcircuit can monitor the voltage across the power storage device andcause charging of the power storage device to cease if the monitoredvoltage is more than the turn off voltage. The dashed line 490represents a voltage across the power storage device after theprocessing circuit has caused charging of the power storage device tocease.

If the voltage across the power storage device is allowed to increaseabove the maximum rated voltage of the power storage device, the powerstorage device could be damaged. The sampling period for sampling thevoltage across the power storage device and the turn off voltage can beselected so as to avoid damaging the power storage device. For a givenmaximum rated voltage of a power storage device and turn off voltage,the maximum time between samples (shown as Tmax2 in FIG. 3D) is the timerequired for the voltage across the power storage device to increasefrom the turn off voltage to the maximum rated voltage of the powerstorage device.

Example Process for Operating the Power Control Circuit

FIG. 4 is a flow diagram of an example process 500 for operating thepower control circuit 304. A local power level is determined (e.g., thepower storage device voltage is read) (502). For example, the processingcircuit 402 of FIG. 3A can read the voltage across the capacitor 406 or,for further example, the microprocessor 462 of FIG. 3B can read thevoltage across the capacitor 453.

The voltage level is compared to a turn off voltage (504). For example,the turn off voltage can be 2.7 volts

If the voltage is greater than or equal to the turn off voltage, theposition of the switch is checked (506). For example, the switchfeedback line 412 can be read by the processing circuit 402 to determinethe state of the switch circuit 404. For further example, the voltagefrom the relay pole 458 can be read by the microcontroller 462 todetermine the position of the relay.

If the switch is on (i.e., set to allow a capacitor to charge) then theswitch is turned off (508). If the switch is not on (i.e., already off)the process ends (510). The switch control line 410 can be used tochange the state of the switch circuit if needed. For further example,the relay of FIG. 3B can be actuated using current flow between the setsof three microcontroller pins to turn it off. If the relay is alreadyoff, the relay is not actuated so as to conserve power (as compared toan open loop process without relay position feedback wherein the relayis directed to the off position if the capacitor voltage is greater thanor equal to the turn off voltage regardless of the current state of therelay).

If the monitored voltage is less than the turn off voltage, themonitored voltage is compared to a turn on voltage (512). For example,the turn on voltage can be 2.5 volts.

If the monitored voltage is greater than a turn on voltage, the processends (514). If the monitored voltage is not greater than the turn onvoltage, the position of the switch is checked (516). For example, theprocessing circuit 402 can read the switch feedback line 412 or themicrocontroller 462 can read the voltage at the pin connected to therelay pole 458.

If the switch is not on, it is turned on (520). For example, theprocessing circuit 402 can direct the switch to change states using theswitch control line 404 or the microcontroller 462 can actuate the relayhaving coil 454.

If the switch is already on, the process ends (518).

The process 500 can be performed, for example, periodically by theprocessing circuit 402 (or the microcontroller 462). When the process isnot being performed, the processing circuit 402 (or the microcontroller462) can wait in a low power state to conserve energy.

Example Process for Updating a Conference Room Schedule

FIG. 5 is a flow diagram of an example process 600 for updating aschedule for a conference room. For convenience, the process 600 will bedescribed with reference to the low power display system 100 of FIG. 1,which can perform the process.

A low power display device is provided in proximity to the conferenceroom (602). For example, the low power display device 130 can be locatedoutside the conference room, e.g., by attaching the low power displaydevice 130 with Velcro to the conference room door or wall.

The low power display device is powered based on local environmentalconditions (604). In some implementations, the low power display deviceis powered by an environmental power source, e.g., indoor solar energyor radio frequency energy.

Schedule information for the conference room is retrieved from a remotesource (606). For example, the low power display device can transmit arequest for calendar data to a base station (e.g., a base station 120)or directly to a server running a calendar application.

Display of the schedule information for the conference room is managedon the low power display device (608). In some implementations, theremote source (e.g., a base station 120) generates instructions forrendering the schedule information and transmits the instructions to thelow power display device.

Embodiments of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Embodiments ofthe subject matter described in this specification can be implemented asone or more computer program products, i.e., one or more modules ofcomputer program instructions encoded on a tangible program carrier forexecution by, or to control the operation of, data processing apparatus.The computer-readable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more of them.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file in a file system. A program can bestored in a portion of a file that holds other programs or data (e.g.,one or more scripts stored in a markup language document), in a singlefile dedicated to the program in question, or in multiple coordinatedfiles (e.g., files that store one or more modules, sub-programs, orportions of code). A computer program can be deployed to be executed onone computer or on multiple computers that are located at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto-optical disks, or optical disks. Computer-readablemedia suitable for storing computer program instructions and datainclude all forms of non-volatile memory, media and memory devices,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described is this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), e.g., the Internet.

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or of what may be claimed, but rather as descriptions offeatures that may be specific to particular embodiments of particularinventions. Certain features that are described in this specification inthe context of separate embodiments can also be implemented incombination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Particular embodiments of the subject matter described in thisspecification have been described. Other embodiments are within thescope of the following claims. For example, the actions recited in theclaims can be performed in a different order and still achieve desirableresults. As one example, the processes depicted in the accompanyingfigures do not necessarily require the particular order shown, orsequential order, to achieve desirable results. In certainimplementations, multitasking and parallel processing may beadvantageous. In some implementations, the low power display system doesnot include base stations, and the low power display devices aredirectly connected to one or more servers.

1. A method, comprising: periodically sampling, by a processing circuit,a voltage of a power storage device; determining, by the processingcircuit and at a first time, that the voltage is less than a turn onvoltage; and in response to determining that the voltage is less thanthe turn on voltage; causing the power storage device to enter acharging state, the power storage device being charged by anenvironmental energy source while in the charging state; periodicallysampling the voltage of the power storage device while the power storagedevice is in the charging state, the periodic sampling having a firstsampling period that is less than an amount of time required for thevoltage of the power storage device to increase from a turn off voltageto a maximum rated voltage of the power storage device, the turn offvoltage being a voltage at which the power storage device istransitioned out of the charging state; and while in the charging state,causing the processing circuit to transition to a low power statebetween each periodic sample of the voltage.
 2. The method of claim 1further comprising: determining, by the processing circuit and at asecond time, that the voltage is greater than the turn off voltage; inresponse to determining that the voltage is greater than the turn offvoltage: causing the power storage device to enter a discharging state;periodically sampling the voltage of the power storage device while thepower storage device is in the discharging state, the periodic samplinghaving a second sampling period that is less than an amount of timerequired for the voltage of the power storage device to decrease fromthe turn on voltage to a minimum input voltage of a power supply poweredby the power storage device; and while the power storage device is inthe discharging state, causing the processing circuit to transition to alow power state between each periodic sample of the voltage.
 3. Themethod of claim 2, wherein the power supply is a charge pump.
 4. Themethod of claim 2, wherein the turn on voltage and the turn off voltageare separated by a hysteresis range of at least 0.2 volts and notransition occurs if the voltage of the power storage device is withinthe hysteresis range.
 5. The method of claim 1, wherein the powerstorage device is a capacitor and the environmental energy source is aphotovoltaic cell.
 6. The method of claim 1, further comprisingproviding power from the power storage device to an electrically powereddevice while the processing circuit is in the low power state.
 7. Apower control circuit, comprising: a capacitor; and a processing circuitconfigured to: periodically sample a voltage across the capacitor;determine, at a first time, that the voltage across the capacitor isgreater than a turn off voltage; and in response to determining that thevoltage is greater than the turn off voltage: cause the capacitor totransition to a discharging state; periodically sample the voltageacross the capacitor while the capacitor is in the discharging state,the periodic sampling having a first a sampling period that is less thanan amount of time required for the capacitor to discharge from a turn onvoltage to a minimum input voltage, the minimum input voltage being avoltage required for operation of a power supply drawing power from thecapacitor, the turn on voltage being a voltage at which the capacitor istransitioned to a charging state; and while the capacitor is in thedischarging state, transition to a low power state between each periodicsample of the voltage across the capacitor.
 8. The power control circuitof claim 7, wherein the processing circuit is further configured tocause the capacitor to transition to the charging state if the voltageacross the capacitor is less than the turn on voltage.
 9. The powercontrol circuit of claim 7 wherein the processing circuit reads samplesthe voltage across the capacitor at least four times per second.
 10. Thepower control circuit of claim 7, wherein the processing circuit ispowered by the power supply drawing current from the capacitor and isconfigured to default to causing the capacitor to transition to thecharging state in the event of loss of power from the power supply. 11.The power control circuit of claim 7, wherein the processing circuitcomprises a microcontroller.
 12. The power control circuit of claim 7,further comprising a switch circuit controlled by the processingcircuit, the switch circuit having an on position and an off position,wherein the switch circuit connects an environmental power source to thecapacitor to charge the capacitor when the switch circuit is in the onposition, and wherein the switch circuit disconnects the environmentalpower source from the capacitor when the switch circuit is in the offposition.
 13. The power control circuit of claim 12, wherein the switchcircuit comprises a relay.
 14. The power control circuit of claim 12,wherein the switch circuit comprises a bistable relay.
 15. The powercontrol circuit of claim 12, wherein: the switch circuit is a bistablerelay; the processing circuit is a microcontroller; and themicrocontroller actuates the relay by passing current from one set ofmicroprocessor pins through a coil of the relay to a second set ofmicroprocessor pins.
 16. The power control circuit of claim 12, whereinthe switch circuit comprises a MOSFET.
 17. The power control circuit ofclaim 7, wherein the processing circuit is further configured to:determine, at a second time, that the voltage across the capacitor isless than the turn on voltage; in response to determining that thevoltage across the capacitor is less than the turn on voltage: cause thecapacitor to transition to a charging state; periodically sample thevoltage across the capacitor while the capacitor is in the chargingstate, the periodic sampling having a second sampling period that isless than an amount of time required for the voltage across thecapacitor to increase from the turn off voltage to a maximum ratedvoltage of the capacitor.
 18. The power control circuit of claim 7,wherein the power supply comprises a power output configured to providepower to an electrically powered device, and wherein the power supplyprovides power to the power output while the processing circuit is inthe lower power state.
 19. A power control circuit, comprising: acapacitor; and a processing circuit configured to: periodically sample avoltage across the capacitor; determine, at a first time, that thevoltage across the capacitor is less than a turn on voltage; and inresponse to determining that the voltage across the capacitor is lessthan the turn on voltage: cause the capacitor to transition to acharging state, the capacitor being charged by an environmental energysource while in the charging state; periodically sample the voltageacross the capacitor while the capacitor is in the charging state, theperiodic sampling having a first a sampling period that less than anamount of time required for current from the environmental energy sourceto charge the capacitor from a turn off voltage to a rated voltagecapacity of the capacitor, the turn off voltage being a voltage at whichthe capacitor is transitioned out of the charging state; and while thecapacitor is in the charging state, transition to a low power statebetween each periodic sample of the voltage across the capacitor. 20.The power control circuit of claim 19, wherein the processing circuit isfurther configured to cause the capacitor to transition to a dischargingstate if the voltage across the capacitor is more than the turn offvoltage.
 21. The power control circuit of claim 19 wherein theprocessing circuit samples the voltage across the capacitor at leastfour times per second.
 22. The power control circuit of claim 19,wherein the processing circuit receives power from a power supplydrawing current from the capacitor and is configured to cause thecapacitor to transition to the charging state if power from the powersupply is lost.
 23. The power control circuit of claim 19, furthercomprising a power supply connected to the capacitor that provides powerto an electrically powered device from the capacitor while theprocessing circuit is in the lower power state.
 24. The power controlcircuit of claim 19, wherein the processing circuit is furtherconfigured to: determine, at a second time, that the voltage across thecapacitor is greater than the turn off voltage; in response todetermining that the voltage across the capacitor is greater than theturn off voltage: cause the capacitor to transition to a dischargingstate; periodically sample the voltage across the capacitor while thecapacitor is in the discharging state, the periodic sampling having asecond sampling period that is less than an amount of time required forthe voltage across the capacitor to discharge from the turn on voltageto a minimum input voltage, the minimum input voltage being a voltagerequired for operation of a power supply drawing power from thecapacitor; and while the capacitor is in the discharging state,transition to a low power state between each periodic sample of thevoltage across the capacitor.